1. Field of the Invention
The present invention relates generally to a mobile communication system, and a mobile station and a base station to be employed therein. More particularly, the invention relates to a transmission power control system in a code divided multi access (CDMA) cellular system, to which soft hand over is applied.
2. Description of the Related Art
In a code divided multi access (CDMA) cellular mobile telephone system, a base station transmission power control technology for controlling a transmission power of a signal to be transmitted from a base station equipment on the basis of a power control information from a mobile station equipment, is employed. FIGS. 30 and 31 are illustrations respectively showing control blocks of a mobile station and a base station for explaining the conventionally typical base station transmission power control method in the CDMA cellular mobile telephone system.
At first, discussion will be given for a control for transmitting the power control information from the mobile station equipment with reference to FIG. 30. In the mobile station, a transmission signal to be transmitted from the base station is received and demodulated by a transmission/reception common unit 1, an RF portion 2 and a demodulation 3, and a reception quality Rq [dB] is measured by a reception quality measuring device 4. Here, the reception quality is measured as a ratio of a received power of the base station transmission signal and unwanted signal power, such as interference and so forth, or a ratio of a received power of the base station transmission signal and an arbitrary reference received power constant.
The received reception quality Rq is compared with a required quality Rth by a comparing portion 14. When the reception quality Rq is greater than the required quality Rth, a transmission power control bit is set at “0” in a processing portion 15. Otherwise, the transmission power control bit is set at “1” by a processing portion 16. The set transmission power control bit is inserted in a transmission signal by a transmission power control bit inserting portion 17. Also, an overhead symbol containing various control information and so forth is inserted in an overhead inserting portion 10.
A transmission data containing the transmission power control bit and an overhead information is transmitted to the base station via a spreading portion 9, a modulator 8 and an amplifier device 7 of the mobile station equipment.
Next, discussion will be given for a transmission power control in the base station on the basis of the power control information from the mobile station with reference to FIG. 31. A reception signal including various data or various control information from the mobile station is received and demodulated via a transmission/reception common unit 21, an RF portion 22, a down converter 23, a despreading portion 24 and a reception data demodulator 25. Then, the transmission power control bit is extracted in the transmission power control bit detector 31.
The extracted transmission power control bit is judged whether the value thereof is “1” or not in a processing portion 32. If the transmission power control bit is “1”, a fixed transmission power control amount ΔP is added for a current transmission power control signal Pctl in a processing portion 33. Otherwise, the fixed transmission power control amount Δ P is subtracted. A transmission spreading RF signal 28 is amplified by a variable amplifier 29 taking the current transmission power control signal Pctl as a control value and transmitted to the mobile station via the transmission/reception common unit 21.
As set forth above, by the transmission power control according to the transmission power control block diagram, the base station transmission power control, in which the reception quality in the mobile station becomes Rth, can be achieved.
In addition to the foregoing transmission power control system (hereinafter referred to as “system A”), some transmission power control systems have been invented. For example, in a transmission power control system (hereinafter referred to as “system B”) recited in Kikuchi, Higashi, Ono, Technical Report of the Institute of Electronics, information and Communication Engineers, RCS96-13, May, 1996, pp 34, right column, links 1–15, there has been proposed a method, in which all of the base station collects communication quality information of all of the connected mobile stations to determine a transmission power unitarily so as to achieve a required quality. By this method, developing of the transmission power can be completed at high speed to restrict occurrence of excessive transmission power during a period required for developing the transmission power.
On the other hand, in the transmission power control system (hereinafter referred to as “system C”) recited in Hamabe, Yoshida, Ushirokawa, Technical Report of the Institute of Electronics, Information and Communication Engineers, RCS-84, August, 1996, pp 126, Section 2.1, there has been proposed a method, in which distribution amounts of the transmission power of the base station for respective mobile stations are determined by the mobile stations on the basis of a pilot reception power, and the base station unitarily determines the transmission power for respective mobile stations on the basis of the distribution information. Instead of seeking for the transmission power control system achieving the reception quality at the desired quality in respective mobile stations as in the systems A and B, this system C is directed only to make the reception quality of all of the mobile stations uniform. Accordingly, while interference restriction effect is considered to be lower in comparison with the system A, high precision and stable control can be performed as system makes reference to the pilot signal transmitted with high power.
In the cellular mobile communication, hand over, in which connected base stations are switched, is effected according to movement of the mobile station. The mobile station in the CDMA cellular mobile communication system, in which all base stations use the same frequency simultaneously, interference for other radio link has to be minimized by constantly connecting to the base station, at which a propagation loss becomes minimum (primary base station), and by performing transmission power control. However, since certain period is required for hand over to new base station, delay in connection with the primary base station can be caused to encounter a drawback in that communication with excessive power is inherent. Therefore, a method that the mobile station is connected to a plurality of base stations to preliminarily take the primary base stations, is employed. This method is referred to as soft hand over (or soft hand off).
Concerning the base station transmission power control during soft hand over, control system disclosed in Japanese Unexamined Patent Publication No. Heisei 9-74378 (hereinafter referred to as “system D”).
In the system D, concerning distribution method of the transmission power per the base station, there has been disclosed three methods, i.e. a method effecting distribution for equalizing the reception powers from respective base stations in the mobile station, a method effecting distribution for making a ratios of the reception power from respective base stations in the mobile station to be equal to the pilot reception level ratio, and a method effecting distribution for making the transmission powers of respective base stations equal to the ratio of the pilot reception level in the mobile station.
On the other hand, a related art relating to the base station transmission power control during soft hand over has been disclosed in commonly owned U.S. patent application Ser. No. 09/090,013. With the disclosed technology, reception qualities of down signals from a plurality of base stations during hand over are monitored in the mobile station and a signal designating the base station to effect transmission according to the result of monitoring is transmitted. Then, in the base station, transmission power control of the down transmission signal to the mobile station is performed according to the base station designation signal.
The conventional transmission power control system for a down link as represented by the foregoing systems A to D, is premised a plural base station transmission, in which a plurality of base stations perform transmission simultaneously during soft hand over. The soft hand over is an essential technology for realizing communication with the base station of the minimum propagation loss in an actual system, in which hand over control delay cannot be ignored. However, concerning the down link, since a plurality of the base stations has to effect transmission for one mobile station, interference to be caused on the mobile station can be increased. Such problem will be discussed with reference to FIGS. 32 and 33.
FIG. 32 is an illustration showing reception signals received by respective mobile stations MS-A and MS-B located in zones Z-A and Z-B respectively covered by the base stations BS-A and BS-B in non-soft hand over state. In the shown condition, the mobile station MS-A is receiving a desired wave of a reception power of PTd-A from the connected base station BS-A and an interference wave of reception power of PTi-B from a non-connected base station BS-B. On the other hand, the mobile station MS-B is receiving a desired wave of a reception power of PTd-B from the connected base station BS-B and an interference wave of reception power of PTi-A from a non-connected base station BS-A. A ratio of the desired wave reception power versus the interference wave reception power observed by the mobile station MS-B becomes PTd-B/PTi-A. Next, similarly to FIG. 32, consideration is given for the base where soft hand over of the mobile station MS-A and the base station BS-B.
FIG. 33 shows the reception signals received by both mobile stations during soft hand over in the mobile station MS-A. The mobile station MS-A in soft hand over receives the desired signal of the reception power of PTd-A from the base station BS-A and the desired signal of the reception power of PTd-A′ from the base station BS-B, namely two desired signals in total. By diversity reception of both desired signal waves in the mobile station MS-A, a diversity gain can be obtained. On the other hand, the signal transmitted by the base station BS-B for soft hand over with the mobile station MS-A, is received as the interference wave in the mobile station MS-B. Assuming that the reception power of the interference wave in the mobile station MS-B is PTi-A′, the ratio of the desired wave reception power versus the interference wave reception power as observed by the mobile station MS-B becomes PTd-B/(PTi-A+PTi-A′). This is smaller than PTd-B/PTi-A of the ratio of the desired wave reception power versus the interference wave reception power during non-sift hand over state set forth above to cause lowering of reception quality by increasing of interference.
Lowering of the reception quality can be compensated in certain extent by also effecting soft hand over in the mobile station MS-B. However, according to M. Soleimanpor and G. H. Freeman, Proceeding of IEEE Vehicular Technology Conference, pp. 1129, right-side column, links 15 to 31, in which increasing amount of interference and the diversity gain by site diversity, increasing amount of interference is greater, As a result, capacity of the down link is restricted. Similar assertion has also be seen in Nakano, Umeda, Ohno, Technical Report of the Institute of Electronics, Information and Communication Engineers, RCS94-100, pp 71, left column, links 1 to 8.
It should be noted that increasing of interference by the plural base station transmission of the down link set forth above will not raise problem in so-called hard hand over in which simultaneous communication between a plurality of base stations and the mobile station is not performed. In this case, influence of hand over period of the base station namely hand over control delay can be a problem. According to Furukawa, Communication Society Conference 1997 of the Institute of Electronics, Information and Communication Engineers, pp. 264, Chapter 3, it is stated that greater transmission power is radiated at greater hand over control delay. Hard hand over under a condition where the hand over control delay cannot be ignored, is inherently performed at an excessive transmission power for the mobile station in formerly connected base station before hand over in order to forcedly establish communication with the formerly connected base station which is currently non-minimum propagation loss base station for the mobile station. As a result, interference for the peripheral mobile stations is increased.
In order to reflect the transmission power control signal from the mobile station in the base station with high fidelity, the transmission power control signal has to be transmitted with small error and small delay. On the other hand, in order to effectively use the communication link, it is desirable that a power of the transmission power control signal is as small as possible. As a technology for reducing reception error, there is a method of interleave, error correction and so forth applied for transmission of the signal of voice, data and so forth. However, this technology is not applied to the transmission power control signal for causing increasing of decoding delay or additional information. As a result, frequency of occurrence of error of the transmission power control signal becomes high in comparison with the data signal. Particularly, upon soft hand over, in the non-primary base station, in which propagation loss is large, reception error becomes large.
When reception error of the transmission power control signal is caused, the transmission power control command from the mobile station cannot be reflected well. Thus, the base station is forced to perform communication at inappropriate transmission power. This will be discussed with reference to FIG. 34. FIG. 34 illustrates variation of the transmission power in time PT-A, PT-B and PT-A′, PT-B′ of respective base station while soft hand over is performed between a certain mobile station and two base stations BS-A and BS-B. In FIG. 34, a vertical axis represents a transmission power [dBW] of base station and a horizontal axis represents a time [sec].
As shown in FIG. 34, when reception error is not caused in the transmission power control, as illustrated by solid links of variation in time transitions PT-A and PT-B, the transmission power of both base stations are maintained at equal value. On the other hand, when reception error is caused in the transmission power control signal as illustrated by dotted link of variation in time transitions PT-A and PT-B, it can be appreciated that lacking of the transmission power is continuously caused in PT-A and excessive transmission power is continuously caused in PT-B. Particularly, when excessive transmission power is continuously caused as in the variation in time transition PT-B, interference is caused for the peripheral cells to reduce capacity to be received in the system.